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function p = dirichletpdf(x, alpha)
%DIRICHLETPDF Dirichlet probability density function.
% p = dirichletpdf(x, alpha) returns the probability of vector
% x under the Dirichlet distribution with parameter vector
% alpha.
%
% Author: David Ross
%-------------------------------------------------
% Check the input
%-------------------------------------------------
error(nargchk(2,2,nargin));
% enusre alpha is a vector
if min(size(alpha)) ~= 1 | ndims(alpha) > 2 | length(alpha) == 1
error('alpha must be a vector');
end
% ensure x is is a vector of the same size as alpha
if any(size(x) ~= size(alpha))
error('x and alpha must be the same size');
end
%-------------------------------------------------
% Main
%-------------------------------------------------
if any(x < 0)
p = 0;
elseif sum(x) ~= 1
disp(['dirichletpdf warning: sum(x)~=1, but this may be ' ...
'due to numerical issues']);
p = 0;
else
z = gammaln(sum(alpha)) - sum(gammaln(alpha));
z = exp(z);
p = z * prod(x.^(alpha-1));
end
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